Heating clean quartz with charcoal in an electric arc furnace removes the oxygen as carbon monoxide and leaves metallurgical silicon, about 98 to 99% pure, which is the raw material and not a solar-cell material. A cell needs at least 99.99% by the carrier’s figure, so this step solves a small part of the problem and leaves the hard part, purification, still to do.
Part of Solar Panel Construction
Carbothermic reduction of quartz to metallurgical-grade silicon, and how far that product is from photovoltaic grade.
The Reaction Is Simple And The Product Is Not Pure Enough
Silicon dioxide plus carbon at high temperature gives silicon and carbon monoxide: SiO₂ + 2C → Si + 2CO (Silicon Purification; the carrier gives the same equation). The molten silicon settles to the bottom and is tapped off. The product is metallurgical-grade silicon, 98–99% pure in both sources, with iron at about 0.3%, aluminium about 0.1% and calcium, titanium and others making up the rest (Silicon Purification).
The two sources disagree on temperature. The carrier asks for the furnace to hold at least 1,900°C, in a furnace “capable of 1,800–2,000°C”; Silicon Purification gives 1,400–1,700°C with silicon melting at 1,414°C. Both agree that an electric arc furnace is needed, and that only the molten silicon tapped from the bottom is the product.
What The Purity Gap Means
Solar cells need silicon of at least 99.99% purity (“four nines”, carrier). The product here is 98–99%, so the impurity content is 1–2% (10,000–20,000 parts per million) against a requirement of 0.01% (100 parts per million). That is a reduction of 100 to 200 times still to come, and a settlement does not yet have the tools to do it: Zone Refining and the routes in Silicon Purification are the next steps. The carrier states that each zone-refining pass improves purity roughly tenfold; whether that holds for the impurities that matter most in silicon (boron and phosphorus) is not shown in the corpus, and general knowledge, not from the other articles, says those two are the hardest to remove this way, so treat the 99.99% claim as unverified.
What You Need
- Clean white quartz or crushed quartz rock; avoid coloured sand, whose impurities pass into the silicon (carrier). Silicon Purification adds that ordinary beach sand is too contaminated and that vein quartz is purer than sedimentary deposits.
- High-quality hardwood charcoal — Silicon Purification advises pre-baking it at 1000°C to drive off volatiles, because charcoal brings potassium, sodium and calcium from the wood (Charcoal Production)
- An arc furnace — Arc Furnace Setup and Carbon Electrode Preparation
- A refractory crucible or hearth and a way to tap and cast the metal (Crucibles)
- A strong electrical supply — at least 10 kW per the carrier
Method: Reduce Quartz To Silicon
Step 1: Crush and wash the quartz to a coarse grit, wet, to keep silica dust down.
Step 2: Mix with charcoal at roughly equal weight (carrier: 1:1 by weight).
Step 3: Charge the furnace and bring it to temperature. Add fresh mix as the charge sinks.
Step 4: Tap the molten silicon from the bottom into a mold, and let it cool. Any slag stays on top.
Step 5: Break up and keep the metal separate from slag and unburnt charge.
Worked Example: How Much Silicon Per Kilogram Of Quartz
The molar masses are general knowledge, not from the other articles: silicon dioxide 60.08 g/mol, silicon 28.09 g/mol, carbon 12.01 g/mol. One mole of SiO₂ gives one mole of Si, so the yield at 100% conversion is 28.09 ÷ 60.08 = 0.467 kg of silicon per kilogram of quartz. The carbon the reaction needs is 2 × 12.01 = 24.02 g per 60.08 g of SiO₂, which is 0.40 kg per kilogram of quartz. The carrier’s 1:1 weight mix therefore carries 2.5 times the carbon the reaction uses, as a margin for the carbon that burns off and the ash it brings. Actual yields are lower than 0.467 kg, and the corpus gives no figure for them.
Common Mistakes
- Treating the product as solar-grade — it is two orders of magnitude from the requirement
- Using coloured sand or dirty charcoal — the impurities go into the silicon
- Skipping the charcoal pre-bake — ash and volatiles contaminate the melt
- Running the furnace in a closed shed — carbon monoxide is the by-product
Safety
The furnace gives off carbon monoxide, UV and molten metal, and crushing quartz makes lung-damaging dust
Each mole of silicon releases two moles of carbon monoxide, a colourless, odourless gas that kills without warning; run the furnace outdoors or under strong extraction. An open arc gives off ultraviolet light that burns the eyes (Carbon Arc Lamp describes arc eye), and molten silicon sits at 1,414°C or above. Crushed quartz dust causes silicosis, an incurable lung disease (Dust & Respiratory Protection), so crush wet and wear a respirator.
What’s Next
- Learn next: Arc Furnace Setup — building the furnace that does this
- Combine with: Zone Refining — the purification this product still needs